Closely spaced bored piles can be used to form a retaining wall, perhaps for the construction of a deep basement or a cut and cover tunnel. The piles may be constructed so that they virtually touch each other (contiguous). The gaps between the piles can be grouted to form a watertight retaining wall.
Alternatively (secant piles) every other pile may be constructed, with their centres less than two diameters apart. In-fill piles are then bored, cutting into the adjacent piles to form a continuous structure. To aid construction, the first sets of piles may be cast with a lower grade of concrete. These may not be load-bearing and act as ‘seals’ between the main load bearing piles.
As the piles interlock, this form of construction leads to a more efficient form of structure. During excavation of the soil, the piles will generally require propping before the permanent floor and/or roof structure are completed.
Because of the form of construction, the exposed piles will be fairly rough in appearance. Thus, in most cases, an inner wall, which may or may not be structural, will be built or some decorative surface applied, e.g. sprayed concrete or cladding. A method of drainage will generally be required between the piles and any inner wall.